Means and method for controlling or preventing oscillations in vacuum tube circuits



l #70 /7/5/ Boyd Pkg/p5 a l+- ((4414 Afforney PHELPS 3 MEANS AND METHODFOR CONTROLLING OR PREVENTING 3 Sheets-Sheet 1 II\ will Filed Oct. 13,1924 OSCILLATIONS IN VACUUM TUBE CIRCUITS Oct. 27, 1931.

3 2 1 t 0 w 9 h 2 RV m N m H S M 3 VS E T RI U m RI 0 C E B. PHELPSMEANS AND METHOD FOR CONTROLLING OSCILLATIONS IN VACUUM TUB Filed Oct.13 1924 [/70'8/7/0!" Boyd Pkg/p5 y w *4 f Oct. 27, 1931.

A #0 racy PHELPS 9,0 3

3 Sheets-Sheet 3 Filed Oct. 13, 1924 MEANS AND METHOD FOR CONTROLLING ORPREVENTING OSCILLATIONS IN VACUUM TUBE CIRCUITS Oct. 27, 1931.

A Home y Patented Oct. 27, 1931 UNITED. STATES PATENT OFFICE.

BOYD PHELPS, 0F GRASMERE, STATEN ISLAND, NEW YORK, ASSIGNOB, BY LS-SIGNMENTS, TO B. E. THOMTSON MANUFACTURING COMPANY, A OOBEORATIOI OI NEWJERSEY arms AND mnrnon iron con'rnoume on. ranvmrrme osma'noxs m vacuumsroan cmcurrs Application filed October 18, 1924. Serial No. 748,268.

My invention relates to vacuum tube circuits; and the same has for itsobject more particularly to provide a simple, efficient and reliableamplifying system in which the effects of undersirable feed back actionare neutralized or prevented.

Further, said invention has for its object to provide a system of thecharacter specified in which undesirable potentials, or the effectsthereof, such as those due to the inter-electrode capacity of the tube,are neutralized, or prevented, without the dissipation of appreciableenergy, and the reduction of amplifying power.

Further, said invention has for its ob ect to provide a system of thecharacter specified which, without adjustment, is effective over itsentire fre uency range in neutralizing or opposing sai undesirable feedback potentials.

Further, said invention has for its object to provide a system of thecharacter specified in which resistance is employed to control the valueof the neutralizing potential so that the latter effects neutralizationof the undesirable feed back potential.

Further, said invention has for its object to provide a system of thecharacter specified in which current from certain of the circuitsthereof is caused to traverse a portion of another circuit, and in whichthe resulting potential drop developed in said portion, as a I result ofthe resistance thereof, is such as to effect the neutralization of theundesirable feed back potentials.

Further, said invention has for its object to provide a novel method forneutralizing or opposing undesirable feed back potentials.

Other objects will in part be obvious and in part be pointed outhereinafter.

To the attainment of the aforesaid objects and ends, my inventionconsists in the several steps comprising the method, and in the novelfeatures of construction, and in the combination, connection andarrangement of parts hereinafter more fully described and then pointedout in the claims.

Fordeveloping the required neutralizing potential, or otential drop, animpedance consisting primarily 'of resistance is eming two stages ofradio frequency am lification, a detector and one stage of au iofrequency amplification.

Fig. 3 is an alternative arrangement with respect to that of Fig. 2.

Fig. 4 is a radio receiving circuit comprising one stage of radiofrequency amplification and a vacuum tube detector.

Fig. 5 is a reflex receiving circuit comprising two stages of radiofrequency amphfication, a crystal detector and three stages of audiofrequency amplificationtwo of the latter being reflexed.

In the clrcuit of Fi 1 vacuum tube 1 is a radio frequency amp er havingan input circuit comprising coil 2 and variable condenser 3 and anoutput circuit comprising coil 4 and plate battery 5. The input circuitis shown associated with an antenna system 6. A filament battery 7 isprovided for heating the filament of the vacuum tube and a rheostat 8 isinserted in series with the battery 7 for adjusting the temperature ofthe filament. The out ut circuit of the amplifier tube may be tuned bythe variable condenser 10, the coil 9 forming a part of the circuit inwhich is included the crystal detector 11 and telephone 12.

Amplifying circuits, such as the sim 1e circuit at Fig. 1 serving as oneexample, because of the inter-electrode capacity of the tube, and othercoupling, and, particularly, when the input and output circuits aretuned to the same frequency feed back energy from one circuit toanother.

The potentials thus fed back are often undesirable, particularly inradio frequen'cy amplifying circuits, as the same tend to causeundesirable oscillation of the tube or tubes of the s stem.

I obviate or neutralize these undesirable feed back potentials by meansof opposing or neutralizing potentials. For this purpose a neutralizinresistance 13 is disposed in a .circuit portion common to both the inputand out ut circuits in series with the input electro es of the tube 1.Since the tube 1 is unidirectional, the current fiows therethrough fromthe plate to the filament thereof, and thereby develops a otential dropbetween the terminals of sai resistance 13 which is such that a negativepotential with respect to the filament is impressed upon the grid. Anyincrease of plate current increases this negative potential and likewiseany de'crease of plate current decreases the negative potential on thegrid. When the resistance 13 has the required value the potentialdeveloped tlierein neutralizes or opposes the undesirable feed backpotential.

The value of neutralizing resistance required to effect completeneutralization depends u on the magnitude of feedback action toovercome. In a circuit such as that shown in Fig. 1, the requiredresistance 80 usually ranges between 5 and ohms and may be conslderablyhigher if the absorption of energy from the amplifier circuit isextremely low. In circuits similar to that of Fig. 1 which have actuallybeen constru'cted, the value of resistance required has been found torange from about 7 to 15 ohms. It is evident that the resistance loss inthe neutralizing resistance is insignificant when the ratio of thisresistance to the total resistance of the output cir'cuit is considered.

The resistance 13, when once adjusted to the correct or balancing value,according to the amount of feed back action in the circuit in which thesame is employed, serves, without further adjustment thereof, toneutralize the undesirable feed back potentials throu hout thewave-length range of the system. 11- der such conditions the gridotential variations due to undesirable feed ack action will be cancelledby the changes in voltage drop across resistance 13. Should the value ofthe resistance employed be increased or decreased from the critical orproper value, it has been found that oscillation of the tube occurs. Forinstance, in actual tests, it has been found that a resistance of thevalue of hundreds of ohms dis osed in the position of the resistance 13(Fig. 1) causes oscillation of the tube, and that the strength of suchoscillation increases as the value of the resistance is increased up toseveral thousand ohms.

When the resistance employed exceeds the value necessary for a balance,the entire s s tem is unstable, the grid potential becoming too muchpositive as the same becomes more ositive and too much negative as thesame comes more negative, the excess of neutralizing potential tendingto cause oscillation.

In like manner, if the value of the resistance is decreased below thecritical or balancing value oscillation of the tube also occurs, becausethere is then an excess of undesirable feed back potential.

It is therefore apparent that the resistance 13 does not prevent theoscillation of the tube because of the damping action thereof, but onthe contrary serves to control the neutralizing potential so that theundesirable feed back potential is neutralized.

In the circuit of Fig. 2 vacuum tubes 14 and 15 are radio frequencyamplifiers;.tube 16 is a detector and tube 17 is an audio frequencyamplifier. In this circuit two neutralizing resistances, 18 and 19, areprovided for neutralizing or controlling undesirable feed backpotentials in the two radio frequency stages. Resistance 18 is includedin the plate filament circuit of tube 14, in the same manner asresistance 13 in Fig. 1. Resistance 19 is likewise included in the platefilament circuit or output circuit of tube 15. These neutralizingresistances are operable, independently, to produce the required varyingpotential drop to be impressed upon the grids of their re-- spectivelyassociated tubes. Coils 20 and 21 having sufficient inductancesubstantially to prevent the passage of radio frequency current, areinserted in the filament battery leads interconnecting the filaments ofthe two radio frequency tubes. By virtue of these coils the highfrequency current in the output circuit of tube 14 is precluded fromflowing through neutralizing resistance 19 and likewise the highfrequency current in the output circuit of tube 15 is precluded fromflowing through neutralizing resistance 18. The neutralization of eachtube is therefore maintained independent of the other. The inclusion ofcoils 20 and 21 is not strictly essential because the circuit can beadjusted to operate satisfactorily without them, but their use isdesirable in the circuit shown in order to prevent upsetting of theneutralization in one or both radio frequency tubes, upon interchanging0i unlike tubes. Further, more exact neutralization can be readilyobtained when the coils 20 and 21 are provided.

With the circuit of Fig. 2 it has been found necessary to provide twoindependent filament heating batteries, 22 and 23, in order to precludefeeding back of audio frequency 2 in that it comprises two stages ofradio frequency amplification, a detector, and one stage 0 audiofrequency amplification. In this circuit the vacuum tubes are designated14', 15', 16 and 17 and these correspond with vacuum tubes 14, 15, 16and 17 of Fig. 2. In the arrangement of Fig. 3 the necessity forproviding two independent filament batteries has been obviated. In thiscase the plate battery 24 is connected to the plates of the radiofrequency amplifier tubes, through high inductance paths including radiofrequency choke coils 25 and 26 respectively, each of which is designedto offer high impedance to both radio frequencies and audio frequencies.Condensers 29 and 30 are rovided to block the flow of direct current trough the primary windings of the radio frequency transformers, 31 and32. Each condenser should be of sufficient size to permit the flo-w ofradio frequency current of the order of frequency to be dealt with.Neutralizing resistances l8 and 19 are included in the output circuitsof their respectively associated vacuum tubes 14' and 15'. It will benoted that onl radio fre uency currents can flow through t eneutralizing resistances in this instance and that there is no pathwhereby audio frequency currents from the detector or audio frequencyStages can find their way to the neutralizing resistances and hence thatthere is no possibility of modulation occurring due to audiofrequencyfeedback. As between Figs. 2 and 3 the latter is of course the preferredembodiment because of the absence of necessity for providing twoindependent filament batteries. The elements shown in Figs. ,2 and 3 andnot specifically described are regarded as being so well known in theart that no'explanation of their function is necessary in order toconvey a clear understanding of the invention and its operation.

The magnitude of neutralizing resistance, which it has been foundnecessary to employ in the circuits of Figs. 2 and 3 usually rangesbetween 7 and 15 ohms for each element.

A' further advantage inherent in the arrangement of Fig. 3, as comparedwith those of Figs. 1 and 2 arises from the fact that the batterycurrent does not flow through the neutralizing resistances whichtherefore do not impress any normal bias on their respective grids. If agrid bias is desired in the circuit of Fig. 3 it may be provided in theusual manner, that is by the addition of a grid battery.

The diagram of Fig. 4 represents a receiving circuit comprising onestage of radio frequency amplification and a vacuum tube detector. Theembodiment of the invention disclosed in this circuit is somewhat of adeparture from those disclosed in the previous figures in that the radiofrequency energy utilized for developing the neutralizing potential dropinstead of being taken directly from the plate circuit of the radiofrequency amplifier is taken from the input circuit of the nextsucceeding tube, which in this in-- stance happens to be a detectortube. Tracing the circuit of the neutralizing resistance 33 it will beseen to include the secondary winding of the radio frequency transformer34, grid condenser 35 in parallel with the grid leak resistance 36 andthe grid and filament of detector tube 37. The flow of radio frequencycurrent in this circuit depends upon the grid filament capacity in thedetector tube. The windings of the transformer 34 are shifted asindicated so that the flow of radio frequency current in the circuit ofthe neutralizing resistance is in the proper direction. It is found thatthis circuit arrangement works equally as well as any of those.

previously described and it has the inherent advantages that there is nonormal grid bias due to the battery current and no choke coils arerequired. The same or a similar arrangement to that shown in Fig. 4 maybe utilized in circuits employing a larger number of vacuum tubes. It isthought that the adapt- 4 ability of this idea to other circuits isreadily apparent.

The diagram of Fig. 5 illustrates an adaptation of the present inventionto a circuit of the reflex type. This circuit includes two refiexedtubes 38 and 39, a crystal detector 40, and a non-flexed audio frequencyamplifier 41. In this System42, 43 and 44 are radio frequencytransformers. The received signals are amplified successively by theradio frequency amplifiers and finally passed through transformer 44 tothe circuit of crystal detector 40, wherein they are rectified and thenpassed through the first audio frequency transformer 45 to the inputcircuit of vacuum tube 38, which functions as both the first radio andfirst audio amplifier.

The amplified audio frequency signals are passed from the output circuitof tube 38 through audio frequency transformer 46 to the input circuitof am lifier tube 39, where they are again ampli ed and then passedthrough audio frequency transformer 47 to the input circuit of the audioamplifier 41 in which the signals are still further amplified. A jack 48is shown in the output circuit of tube 41 for the usual loud speakerconnection.

To the extent thus far described. the circuit of Fig. 5 is of theordinary reflex type. A neutralizing resistance element 49 is showninserted in a circuit including the plate and filament electrodes of thetube 39,

. the primary Winding of transformer 44 and ing of transformer 45. Theradio frequency current flowin in the output circuit of tube that thesingle neutralizing resistance 49, re-

ceiving its energy from the output circuit of tube 39 and impressing itsderived potential difference across the grid and filament electrodes oftube 38, is effective to neutralize the feedback action in both radiofrequency amplifiers. It is presumed that the neutralizing potentialimpressed upon the grid of tube 38 is impressed in amplified form uponthe grid of tube 39. Whether or not this is a correct explanation isuncertain and whether or not exact neutralization of both radiofrequency amplifier tube is effected is like-. wise uncertain. It is,however, entirely certain that the generation of oscillations in bothradio frequenc amplifier tubes can be effectively prevente by the use ofthe single neutralizing resistance as shown. Likewise regeneration canbe completely eliminated in both radio frequency stages. Whether or notthis involves a slight over-neutralization of one or the other radiofrequency amplifiers has not been determined, but at any rate the degreeof amplification is not preceptibly diminished over that obtainable inthe radio frequency stages of the circuits previously describedutilizing separate neutralizing resistances for each radio frequencystage.

In the circuit of Fig. the primary of the transformer 47 serves as ahigh inductance or choke, like the elements and 26 at Fig. 3, in serieswith the plate battery, and offering a high impedance to radiofrequencies.

The resistance 49 is in a high frequency circuit arranged exteriorly ofand in parallel with the portion 'of the late circuit including saidbattery and sai primary. The condenser 50 in the radio fre uency circuitserves to block the flow of direct current through the resistance 49,and hence no normal bias is impressed by the battery upon the grid ofthe tube 38.

It is apparent that the invention herein described is capable of variousmodifications and that its use is not limited to the articular types ofcircuits herein describe Furthermore the invention is not limited to usein conjunction with radio frequency amplifiers, but is equallyapplicable to audio frequency amplifiers wherever there is any tendencytoward excessive regeneration or audio frequency howling in suchamplifiers.

I claim:

1. The hereindescribed method of stabilizing a vacuum tube amplifierhaving an in- .consists in taking a signaling herent tendency towardsself oscillation due to an undesirable feed-back of energy, whichfrequency current from the output side 0 a tube thereof, causing thesame to traverse a resistance, thereby developing a otential differencewhich fluctuates at signa ing frequency, and impressing said potentialdifference fluctuatingat signaling frequenc across the input electrodesof a vacuum tu e in opposition to the potential difference impressedacross said input electrodes due to the feed-back action.

2. The hereindescribed method of stabilizing a vacuum tube amplifierhaving an inherent tendency towards self-oscillation at high frequencydue to an undesirable feedback of energy, which consists in taking ahigh frequency current correspondin to that in circuits at the outputside of a tu causing the same to traverse a resistance, therebydeveloping a potential difference which fluctuates synchronously withand proportionately to said high frequency current, and impressing atleast a part of said fluctuating potential difference across the inputelec-' trodes of a vacuum tube to effectively oppose at every instantthe corresponding high frequency potential difference impressed acrossthe electrodes as a result of said feed-back action.

3. The hereindescribed method of stabilizing a radio frequency vacuumtube amplifier having an inherent tendency towards self oscillation athigh frequencies due to an undesirable feed-back of energy, whichconsists in taking a high frequency current from the output circuit ofone tube, causing the same to traverse a portion of the input circuit ofanother tube, thereby developing a potential difference which fluctuatessynchronously with and proportionately to said radio frequency current,and impressing said fluctuating potential difference across the inputelectrodes of said second named tube to effectively oppose at everyinstant the correspending radio frequency potential difference impressedacross said electrodes as a result of said feed-back action.

4. A high frequency amplifier comprising a vacuum tube having circuitsat the input and output sides thereof including high frequency portions,and aresistance included in circuit with certain of the high frequencyportions at the output side of. said tube and traversed by the hlghfrequency current to develop a corresponding hlgh frequency potentialdifference across said resistance; sald resistance being included incircuit with the input electrodes of said tube for impressing said highfrequency potential difference across the same in opposition to thecorre sponding high frequency potential difference impressed across thesame as a result of undesirable feed-back action.

5. A high frequency amplifier comprising a vacuum tube having circuitsat the in ut and output sides thereof including high requency portions,and a resistance included in circuit with certain of said high frequencyportions at the output side of said tube and traversed by high frequencycurrent to develop a corresponding high frequency potential differenceacross said resistance; said resistance being included in circuit withthe in ut electrodes of said tube for impressing said high frequencypotential difference across the same, and adjusted to effectively opposeat every instant the corresponding radio frequency potential differencesimpressed across said electrodes as a result of undesirable feed-backaction.

' 6. A high frequency amplifier comprising a vacuum tube having circuitsat the in ut and output sides thereof including radio requency portionsand having a coupling tending to cause self oscillation due toregenerative feed-back action; the input electrodes of said tube havingresistance in series therewith connected in circuit with certain of saidhigh frequency portions at the output side of said tube and traversed byhigh frequency current to develop a corresponding high frequencypotential difference across said resistance; and said coupling and saidresistance being relatively adjusted to impress across said inputelectrodes a high frequency potential difference having a value toeffectively oppose at every instant the corresponding high frequencypotential difference impressed across said electrodes as a result ofsaid feed-back action.

7. A multi-stage high-frequency vacuum tube amplifier comprising aplurality of vacuum tubes coupled in cascade, each of said vacuum tubeshaving an input circuit and an output circuit, an ohmic resistance and acondenser in series, circuit connections including said ohmic resistanceand condenser whereby highirequency current derived from andcorresponding to the high-frequency current in certain of said outputcircuits is caused to traverse said ohmic resistance and said condenserin series, whereby a corres onding high-frequency potential difference1s developed across said ohmic resistance, and circuit connections forimpressing said developed high-frequency potential difference across theinput electrodes of certain of said vacuum tubes to effectively opposeany high-frequency potential difference which may be impressed acrosssaid input electrodes as a result of undesirable feed-back action.

8. A multi-stage high-frequency vacuum tube amplifier comprising aplurality of vacuum tubes coupled in cascade, each of said vacuum tubeshaving an input circuit and an output circuit, an ohmic resistance and acondenser in series, circuit connections including said ohmic resistanceand condenser whereby high-frequency current derived from andcorresponding to the highuency current in certain of said outputcircuits is caused to traverse said ohmic resistance and said condenserin series whereby a corres nding high-frequency potential difference 18developed across said ohmic resistance, and circuit connections forimpressing said developed high-frequency potential difference across theinput electrodes of certain of said vacuum tubes to effectively opposeany hi h-frequency potential difference which may impressed across saidinput electrodes as a result of undesirable feed-back action; saidcondenser being effective to regulate the impedance of the circuitincludin said ohmic resistance proportionately to t e frequency of operation, thereby automatically increasing the opposing effect as thetendency toward selfosc-illation increases.

9. An amplifying system comprising a vacuum tube having radio frequencycircuits at the input and output sides thereof in operative relation tosaid tube, a source of space current for operating the system, and aresistance common to the input circuit of said tube and a radiofrequency circuit at the opposite side of said tube and traversed byhigh frequency currents for developing a high frequency potentialdifference across said resistance and impressing the same across theinput electrodes of said tube in opposition to the corresponding highfrequency potential difference impressed across said electrodes as aresult of undesirable feed back action, said resistance being disposedexteriorly of the circuit traversed by the space current to exclude thelatter therefrom.

10. An amplifying system comprising a vacuum tube having radio frequencycircuits at the input and output sides thereof in operative relation tosaid tube, a source of space current for operating the system, and aresistance common to the input circuit of said tube and a radiofrequency circuit at the opposite side of said tube and traversed byhigh frequency current for developing a high frequency potentialdifference across said resistance and impressing the same across theinput electrodes of said tube in opposition to the corresponding highfrequency potential difference impressed across said electrodes as aresult of undesirable feed back action, said resistance being disposedexteriorly of the circuit traversed by the space current to exclude thelatter therefrom, and means serving to exclude radio frequency currentfrom the circuit traversed by space current.

11. An amplifying system comprising a vacuum tube having radio frequencycircuits at the input and output sides thereof, a source of spacecurrent therefor, a resistance in the input circuit of said tube and ina high frequencycircuit at the opposite side of said tube in shunt withsaid source of space current, said resistance developing a high pressedacross sai undesirable feed-back action, an mductance frequencypotential difference and impresthe same across the input electrodes ofsai tube in opposition to the corresponding high frequency otentialdifference imelectrodes as a result of for excluding radio frequencycurrent from the circuit of said source of s ace current, and a capacityfor excluding e space current from said resistance.

12. An amplifying system comprising a plurality of vacuum tubes, in utand output circuits-for said tubes inclu ing radio frequency portions,and impedance means associated with the input circuit of one tube andwith the radio frequency ortion of the output circuit of another tu e,and serving to impress a radio frequency potential drop between theinput electrodes of certain of said tubes in opposition to thecorresponding potential drop impressed therebetween as a result ofundesirable feed back action.

13. An amplifying system comprising a plurality of vacuum tubesassociated in tandem, input and output circuits for said tubes havingradio frequency portions, and a neutralizing resistance included in theinput .circuit of one of said tubes and in the radio frequency portionsof the output circuit of another of said tubes; the arrangement beingsuch that currents in said last mentioned output circuit are caused totraverse said neutralizing resistance, and that the radio frequencypotential drop thereby developed is impressed across the inputelectrodes of at least one of said tubes to oppose the correspondingotential drop resulting from undesirable eed-back action.

14. An amplifying system comprising a plurality of vacuum tubesassociated in tandem, input and output circuits for said tubes, aneutralizing resistance element in the input circuit of one of saidtubes and in the output circuit of another of said tubes, a source ofspace current arranged exteriorly of the portion of said output circuitincluding said resistance element, an inductance in series with saidsource of space current exteriorly of the portion of said output circuitincluding said resistance element, and a capacity in saidlast namedportion of said output circuit; said resistance element serving toimpress a potential drop between the input electrodes of certain of saidtubes in opposition to the potential drop impressed between saidelectrodes as a result of undesirable feed back action, and having avalue such as to effect substantially complete neutralization of saidlast mentioned potential drop.

15; A high frequency amplifier comprising a vacuum tube having circuitsat the input and output sides thereof including radio frequencyportions, ohmic resistance in circuit with certain of said radiofrequency portions at the output side of said tube, and

traversed by the radio frequency current to develop a radio frequencypotential difference across the same, means between said resistance andthe input electrodes of said tube for im ressing said radio frequencypotential di erence between said electrodes 1n opposition to thecorresponding radio frequency potential difference impressed between thesame as a result of undesirable feed-back action, and means in circuitwith said resistance for controlling the same in accordance with thefrequency to automatically vary the effect thereo as the feedbackvaries. 16. A high frequency amplifier comprislng a vacuum tube havingcircuits at the input and output sides thereof including radio frequencyportions, a source of space current in said output circit, ohmicresistance and a condenser in series in circuit with certain of saidradio frequency portions at the output side of said tube, and traversedby the radio frequency current to develop a correspondmg rad1ofrequencys potential difference across the same; said condenser servingto exclude said space current from said resistance and to control thesame in accordance with the frequency. and circuit connections betweensaid resistance and the, input electrodes of said tube for impressingsaid radio frequency potential difference between said electrodes inopposition to the corresponding radio frequency potential differenceimpressed between the same as a result of undesirable feed-back action.

In testimony whereof I afiix my signature.

BOYD PHELPS.

